Problem with light manganese phosphate coating (parkerizing)

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The discussion centers on issues with manganese phosphating for gun parts after switching from manual sandblasting to a Goff 1244 spinner hanger shot blast machine using Chronital S20 stainless steel shot. The new process results in a lighter coating, attributed to a smoother surface finish that may reduce the exposed surface area for phosphating. A microscopic analysis revealed that the shot-blasted surfaces are indeed smoother than those blasted with aluminum oxide. The user has since switched to Grittal GM20 abrasive media, which produces a surface profile similar to aluminum oxide, but still faces challenges with coating adhesion, particularly on hardened steel parts. The potential for residual media coating preventing proper phosphating is a key concern needing further investigation.
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Hello all,

We have a manganese phosphating process for gun parts and I am having trouble with some parts coming out with a lighter coating than normal. Our orginal steps which, for years, have produced repeatable high quality results are; Sand blast with aluminum oxide, phospahte bath at 190°F for 20-30 min, flowing water rinse, water displacing oil bath (WD-40), and then air dry.

What has changed is that we bought a Goff 1244 spinner hanger shot blast machine and are going to use it in place of the manual sandblasting. We are using Chronital S20 Stainless Steel shot. It does a great job at removing any old coatings and oxidation. But when we parkerize parts from this machine, they come out much lighter than when sand blasted.

I am going to set up an alkline bath and rinse to do between blasting and parking.

I want to see if anyone had any thought on this or experience.

Let me know if you have any questions.

Thanks,
Chris
 
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may i suggest a microscopic analysis of the surfaces under the old and new blasting regimes ? I'm thinking the new blasting is not producing as much exposed surface area.
 
That is a great idea. We were just talking about the surface feeling smoother on the rifles coming out of the shotblast machine. I will find a microscope today.
 
awol99 said:
may i suggest a microscopic analysis of the surfaces under the old and new blasting regimes ? I'm thinking the new blasting is not producing as much exposed surface area.

Hello,
So, 10x is the best magnification I could get right now. But even under that the shot blasted surface was much smoother than the sand blasted surface. I have a surface profile gauge on order to start getting some numbers to represent this.

We have removed all the Chronital S20 shot from the spinner hanger and replaced it with Grittal GM20 abrasive media. It is a stainless steel crushed media that is supposed to be comparable to the 50 grit Aluminum Oxide. The blasted surface is VERY similar to that of the aluminum oxide under 10X. But only with a hot water rinse before the phosphate bath we were able to get a dark gray coating. Without the rinse, ONLY the gas tube, takes a coating. It is mild steel ~30 Rockwell. ALL the hardened steel parts ~45 Rockwell (barrel, receiver, sights, gas block) take only a very light gray coating.

I have a load of GM30 Grittal coming tomorrow, slightly coarser.

Anyone have any thoughts on this?

The media is stainless steel, stainless does not parkerize. Could some of the media be leaving some sort of coating on the parts that is preventing them from taking a coating?

I am very stumped by this and very much need help.
 

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